1/(1+ctg^2 a)=1/(1+ cos^2 a/sin^2 a)=1/((sin^2 a+cos^2 a)/sin^2 a)=1/(1/sin^2 a)=sin^2 a
Ответ: Б)
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1/(1+ctg^2 a)=1/(1+ cos^2 a/sin^2 a)=1/((sin^2 a+cos^2 a)/sin^2 a)=1/(1/sin^2 a)=sin^2 a
Ответ: Б)